In marine studies or other activities involving such bodies of water as seas, lakes, rivers, etc, submersible vehicles are often used. In order to obtain samples at exact locations, it is necessary to move the vehicle accurately relative to the floor of the body of water, especially if that vehicle is a remote controlled vehicle (R.O.V.). It is however very difficult to precisely control the movement of such vehicle under water. Particularly so, if there are water currents or other disturbances. In particular there is a requirement for a simple method to take a matched series of photographs of the ocean floor or the like, thereby producing a picture of larger areas of the ocean floor than is presently possible with single frame shot. It is therefore required to produce a mosaic assembly of photographs, similar to, but smaller than those made over land masses by aircraft.
The present way of accomplishing an underwater mosaic is to relate the position of a single frame photograph to a previously established acoustic beacon "lanes" on the ocean bed. Photographs are then taken from a camera assembly that is "flown" close to the ocean bed by towing from a surface ship. The developed photographs are then assembled by relating the position of the frames to previously recorded acoustic data within each "lane". Overlap of each related frames and "lanes" must take place to achieve full coverage. The most difficult problem is keeping the camera in a straight path and at the correct height. Because the camera platform (or support) is towed at a considerable distance from the ship and by a flexible connection (cable or wire), lateral and vertical movement of the camera does take place, thereby distorting the overlaps and photograph size. Most importantly is the fact that the extent of "wondering" is only known after the swing by assembly.
Canadian Pat. No. 892,351 Feb. 8, 1972 (Dessureault) teaches one of many improvements in stabilizing an underwater towed body. THe patent describes pivotally mounted rudders under gravity influence which maintain the towed body at a constant elevation or in a stable vertical plane relative to the towing device.
Another practice of using a bottom sledge platform does generally overcome the altitude problem but due to bottom obstructions it does not prevent lateral movement. Therefore the acoustic marker will give knowledge of the position where the camera has taken photographs, but there is no guarantee that overlap has taken place. In practice, total complete overlap between "lanes" is rarely achieved. A way of overcoming this limitation is to equip the towed body with side thrusters and linking the control of the thrusters to the acoustic marker's position so that "lane wandering" is automatically corrected: similar to automatic blind landing techniques sued by aircraft. Obviously it would become a very expensive and complex system.
While these methods and techniques, with the right environmental factors at the site, and better than average success, will produce a relatively large area mosaic (several hundred feet long), the methods are rarely used due to the expense and cost in time. THere is, however, a greater demand for the production of mosaics in relatively smaller areas at special sites such as oil production areas, bottom investigation of iceberg scours and sand waves, and marine biology and archaeology sites.